Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) are often present together in C8 aromatic product streams from chemical plants and oil refineries. Of these C8 compounds, although EB is an important raw material for the production of styrene, for a variety of reasons most EB feedstocks used in styrene production are produced by alkylation of benzene with ethylene, rather than by recovery from a C8 aromatics stream. Of the three xylene isomers, PX has the largest commercial market and is used primarily for manufacturing terephthalic acid and terephthalate esters for use in the production of various polymers such as poly(ethylene terephthalate), polypropylene terephthalate), and poly(butene terephthalate). While OX and MX are useful as solvents and raw materials for making products such as phthalic anhydride and isophthalic acid, market demand for OX and MX and their downstream derivatives is much smaller than that for PX.
Given the higher demand for PX as compared with its other isomers, there is significant commercial interest in maximizing PX production from any given source of C8 aromatic materials. However, there are two major technical challenges in achieving this goal of maximizing PX yield. Firstly, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamics of production of the C8 aromatic stream in a particular plant or refinery. As a result, the PX production is limited, at most, to the amount originally present in the C8 aromatic stream unless additional processing steps are used to increase the amount of PX and/or to improve the PX recovery efficiency. Secondly, the C8 aromatics are difficult to separate due to their similar chemical structures and physical properties and identical molecular weights.
A variety of methods are known to increase the concentration of PX in a C8 aromatics stream. These methods normally involve recycling the stream between a separation step, in which at least part of the PX is recovered to produce a PX-depleted stream, and a xylene isomerization step, in which the PX content of the PX-depleted stream is returned back towards equilibrium concentration, typically by contact with a molecular sieve catalyst. However, the commercial utility of these methods depends on the efficiency, cost effectiveness and rapidity of the separation step which, as discussed above, is complicated by the chemical and physical similarity of the different C8 isomers.
Fractional distillation is a commonly used method for separating different components in chemical mixture. However, it is difficult to use conventional fractional distillation technologies to separate EB and the different xylene isomers because the boiling points of the four C8 aromatics fall within a very narrow 8° C. range, namely from about 136° C. to about 144° C. (see Table 1 below). In particular, the boiling points of PX and EB are about 2° C. apart, whereas the boiling points of PX and MX are only about 1° C. apart. As a result, large equipment, significant energy consumption, and/or substantial recycles would be required for fractional distillation to provide effective C8 aromatic separation.
TABLE IC8 compoundBoiling Point (° C.)Freezing Point (° C.)EB136−95PX13813MX139−48OX144−25
Fractional crystallization is an alternative method of separating components of a mixture and takes advantage of the differences between the freezing points and solubilities of the components at different temperatures. Due to its relatively higher freezing point, PX can be separated as a solid from a C8 aromatic stream by fractional crystallization while the other components are recovered in a PX-depleted filtrate. High PX purity, a key property needed for satisfactory conversion of PX to terephthalic acid and terephthalate esters, can be obtained by this type of fractional crystallization. U.S. Pat. No. 4,120,911 provides a description of this method. Commercially available fractional crystallization processes and apparatus include the crystallization isofining process, the continuous countercurrent crystallization process, direct CO2 crystallizer, and scraped drum crystallizers. Due to high utility usage and the formation of a eutectic between PX and MX, it is usually more advantageous to use a feed with as high an initial PX concentration as possible when using fractional crystallization to recover PX.
An alternative xylene separation method uses molecular sieves, such as zeolites, to selectively adsorbed para-xylene from the C8 aromatic feedstream to form a PX-depleted effluent. The adsorbed PX can then be desorbed by various ways such as heating, lowering the PX partial pressure or stripping. (See generally U.S. Pat. Nos. 3,706,812, 3,732,325 and 4,886,929). Two commercially available processes used in many chemical plants or refineries are PAREX™ and ELUXYL™ processes. Both processes use molecular sieves to adsorb PX. In such molecular-sieve based adsorption processes, a higher amount of PX, typically over 90%, compared with that from a fractional crystallization process, typically below 65%, may be recovered from the PX present in a particular feed.
For many of these PX separation processes, the higher the original PX concentration in the feed stream, the easier, more efficient and more economical it becomes to perform the PX separation. Therefore, there are strong economic and technical incentives to increase the PX concentration in a hydrocarbon feed stream comprising the C8 aromatic compounds prior to sending the feed stream to a PX recovery unit.
Known technologies integrate selective adsorption and fractional crystallization in PX separation and isomerization loops. For example, U.S. Publication No. 2007/0249882 teaches a process whereby a C8-rich stream is fed to both selective adsorption and fractional crystallization recovery units. By taking advantage of the higher recovery rates of the selective adsorption unit, the PX-depleted effluent (which is typically below 65% depleted) from the fractional crystallization recovery unit can be fed to the selective adsorption unit. While this yields greater efficiency, further advancements were achieved by increasing the C8 concentration of the fractional crystallization PX-depleted effluent stream prior to further recovery by selective adsorption. This was achieved by a xylene isomerization step between the fractional crystallization and the selective adsorption units. Furthermore, liquid-phase isomerization was preferred in this step because of cost, simplicity (no need for hydrogen recycle), and low xylene loss.
The liquid-phase isomerization of C8 aromatics to increase the PX concentration is temperature dependent. That is, the conversion efficiency to PX increases with increasing temperature with the primary controlling limitation being the ability to maintain a liquid state. Although increased system pressure can allow for higher temperatures, physical and cost constraints ultimately place limitations on the process. As a result, liquid-phase isomerization typically operates near 300° C. and at pressures above 300 psig.
While integrated systems of this type yield advantageous efficiencies, the selective adsorption unit is intolerant to C7− hydrocarbons, particularly benzene, and C9+ aromatic compounds (9 or more carbons aromatic compounds). In fact, most selective adsorption units commercially employed can only tolerate up to about 300 ppm of benzene and less than 5000 ppm C9+ aromatic compounds. Thus, the production of C7− hydrocarbons or C9+ aromatics by liquid-phase isomerization processes, particularly those positioned between fractional crystallization and selective adsorption, presents additional problems for overall process efficiency.